This invention relates to fiber optic communication cables, and more particularly to a fiber optic cable structure adapted for well logging for measuring characteristics of earth formations in a borehole.
As an oil well is being drilled, a sonde is usually lowered periodically into the borehole to measure characteristics of the earth formations it traverses. Typically, a logging cable supports and moves the sonde within the borehole, carries power for the sonde, and relays control instructions and data between the sonde and instrumentation and control facilities at the surface of the earth. As measurements and measuring instruments have become more sophisticated, data transmission rates have increased to the point where existing electrical cables can become saturated.
Fiber optic technology can increase data transmission rates several orders of magnitude, as has been demonstrated by fiber optic telephone cables. Due to the demanding conditions under which a well logging cable is used, however, telephonic fiber optic cables would ordinarily not be acceptable. Telephone cables are designed to remain stationary in use and not to encounter the extremes of temperature and pressure found in an oil well.
In contrast, a well logging cable is repeatedly pulled around sheave wheels and wound onto and off a winch drum as it is lowered into and lifted out of wells. The cable must therefore withstand repeated bending around diameters of but a few feet, and tensions of thousands of pounds. Once in the well, the cable encounters pressures which may exceed twenty thousand pounds per square inch and temperatures which may exceed 175.degree. C. Optical fibers, however, are extremely sensitive to deformation (especially point stresses), which greatly increase the attenuation of the optical signals within the fiber. They are also sensitive to moisture, which attacks micro-cracks in the fibers, reduces their strength, and increases their attenuation. While the cable is being manufactured, and later when in use, the stresses (bending and stretching) on the cable components (electrical conductors, strength members, etc.) move them relative to one another within the cable. This can cause local deformations of the optical fibers. Stretching the cable stretches the fibers, thereby increasing their stress, aggravating their attenuation, and sometimes causing them to break. The high pressures and temperatures within the well assist moisture in invading the cable and the optical fibers. As indicated, typical optical telephonic communication cables are not designed for these operating conditions.
Improved optical fiber cable structures have been proposed, but a need still remains for even better buffering for the optical fibers against stresses and better moisture protection in a cable specifically suited for repetitive and demanding well logging applications.